Preparation of 18f-fluciclovine

ABSTRACT

The present invention provides a method for the production of [ 18 F]-FACBC which has advantages over know such methods. Also provided by the present invention is a system to carry out the method of the invention and a cassette suitable for carrying out the method of the invention on an automated radiosynthesis apparatus.

TECHNICAL FIELD OF THE INVENTION

The invention relates to a method for the preparation of aradiopharmaceutical compound, in particular an amino acid derivativeuseful as a positron emission tomography (PET) tracer. The method of theinvention is especially suitable when automated and offers advantagesover known methods. Particularly, the invention relates to a method forpreparation of [¹⁸F]-1-amino-3-fluorocyclobutane-1-carboxylic acid([¹⁸F]-FACBC, also known as [¹⁸F]-Fluciclovine).

DESCRIPTION OF RELATED ART

The non-natural amino acid[¹⁸F]-1-amino-3-fluorocyclobutane-1-carboxylic acid ([¹⁸F]-FACBC, alsoknown as [¹⁸F]-Fluciclovine) is taken up specifically by amino acidtransporters and has shown promise for tumour imaging with positronemission tomography (PET).

A known synthesis of [¹⁸F]-FACBC begins with the provision of theprotected precursor compound1-(N-(t-butoxycarbonyl)amino)-3-[((trifluoromethyl)sulfonyl)oxy]-cyclobutane-1-carboxylicacid ethyl ester. This precursor compound is first labelled with[¹⁸F]-fluoride:

before removal of the two protecting groups:

EP2017258 (A1) teaches removal of the ethyl protecting group by trappingthe [¹⁸F]-labelled precursor compound (II) onto a solid phase extraction(SPE) cartridge and incubating with 0.8 mL of a 4 mol/L solution ofsodium hydroxide (NaOH). After 3 minutes incubation the NaOH solutionwas collected in a vial and a further 0.8 mL 4 mol/L NaOH added to theSPE cartridge to repeat the procedure. Thereafter the SPE cartridge waswashed with 3 mL water and the wash solution combined with the collectedNaOH solution. Then 2.2 mL of 6 mol/L HCl was then added with heating to60° C. for 5 minutes to remove the Boc protecting group. The resultingsolution was purified by passing through (i) an ion retardation columnto remove Na⁺ from excess NaOH and Cl⁻ from extra HCl needed toneutralise excess of NaOH to get a highly acidic solution before theacidic hydrolysis step, (ii) an alumina column, and (iii) areverse-phase column. There is scope for the deprotection step(s) and/orthe purification step in the production of [¹⁸F]-FACBC to be simplified.

SUMMARY OF THE INVENTION

The present invention provides a method for the production of[¹⁸F]-FACBC which has advantages over know such methods. The method ofthe present invention is particularly amenable to automation as itpermits a simplified purification procedure compared with known methods.In the method of the present invention an extra high volume of H′ is notrequired in the Boc deprotection step as it is in the prior art method.Furthermore, an ion removal step such as by means of an ion retardationcolumn such as is required in the prior art method is not required bythe method of the invention because there is no longer a need for excessions to be removed. Also provided by the present invention is a systemto carry out the method of the invention and a cassette suitable forcarrying out the method of the invention on an automated radiosynthesisapparatus.

DETAILED DESCRIPTION OF THE INVENTION

In one aspect the present invention provides a method to prepare1-amino-3-[¹⁸F]-fluorocyclobutanecarboxylic acid ([¹⁸F]-FACBC) whereinsaid method comprises:

-   -   (a) providing a compound of Formula II adsorbed to a solid        phase:

-   -   -   wherein:        -   PG¹ is a carboxy protecting group; and,        -   PG² is an amine protecting group;

    -   (b) reacting said adsorbed compound of Formula II with a PG¹        deprotecting agent;

    -   (c) sending the PG¹ deprotecting agent to waste following said        reacting step (b);

    -   (d) passing an elution solution through said solid phase to        obtain an eluted compound of Formula III:

-   -   (e) reacting said eluted compound of Formula III obtained in        step (d) with a PG² deprotecting agent to obtain a reaction        mixture comprising [¹⁸F]-FACBC.

The “solid phase” used in step (a) of the method of the invention iscontained within a solid phase extraction (SPE) column. Suitably, saidsolid phase is one having a hydrophobic functional group such as phenyl,cyclohexyl and alkyl, for example one having a structure comprising asupport to which C₂₋₁₈ alkyl groups are attached via silicon. In apreferred embodiment, the SPE column is filled with a solid phase havingoctadecylsilyl groups as functional groups. Moreover, it is preferableto use a column packing having a structure in which the functionalgroups are not easily detached from the solid phase under aqueousreaction conditions and/or during a long deesterification reaction. Inone embodiment the SPE column is a tC18 column.

The compound of Formula II is relatively hydrophobic and therefore has astrong affinity for the solid phase and therefore binds to, or becomes“adsorbed”, to said solid phase by virtue of hydrophobic interactions.

The term “protecting group” refers to a group which inhibits orsuppresses undesirable chemical reactions, but which is designed to besufficiently reactive that it may be cleaved from the functional groupin question to obtain the desired product under mild enough conditionsthat do not modify the rest of the molecule. Protecting groups are wellknown to those skilled in the art and are described in ‘ProtectiveGroups in Organic Synthesis’, Theorodora W. Greene and Peter G. M. Wuts,(Fourth Edition, John Wiley & Sons, 2007).

The term “reacting” refers to bringing two or more chemical substances(typically referred to in the art as “reactants” or “reagents”) togetherto result in a chemical change in one or both/all of the chemicalsubstances. For example, in the present invention, the step of reactinga PG¹ deprotecting agent with an adsorbed compound of Formula II changessaid compound of Formula II to a compound of Formula III.

The PG¹ “carboxy protecting group” is preferably linear or branchedC₁₋₁₀ alkyl chain or an aryl substituent. The term “alkyl” used eitheralone or as part of another group is defined as any straight, branchedor cyclic, saturated or unsaturated C_(n)H_(2n+1) group. The term “aryl”refers to any C₆₋₁₄ molecular fragment or group which is derived from amonocyclic or polycyclic aromatic hydrocarbon, or a monocyclic orpolycyclic heteroaromatic hydrocarbon. In one embodiment of the methodof the invention PG¹ is selected from methyl, ethyl, t-butyl and phenyl.In another embodiment of the invention PG¹ is methyl or ethyl and in yetanother embodiment PG¹ is ethyl.

The PG² “amine protecting group” suitably prevents reaction between ¹⁸Fand the amino group in the process of providing the compound of FormulaII. Examples of suitable amine protecting groups include variouscarbamate substituents, various amide substituents, various imidesubstituents, and various amine substituents. Preferably, the amineprotecting group is selected from the group consisting of linear orbranched C₂₋₇ alkyloxycarbonyl substituents, linear or branched C₃₋₇alkenyloxycarbonyl substituents, C₇₋₁₂ benzyloxycarbonyl substituentsthat may have a modifying group, C₂₋₇ alkyldithiooxycarbonylsubstituents, linear or branched C₁₋₆ alkylamide substituents, linear orbranched C₂₋₆ alkenylamide substituents, C₆₋₁₁ benzamide substituentsthat may have a modifying group, C₄₋₁₀ cyclic imide substituents, C₆₋₁₁aromatic imine substituents that may have a substituent, linear orbranched C₁₋₆ alkylamine substituents, linear or branched C₂₋₆alkenylamine substituents, and C₆₋₁₁ benzylamine substituents that mayhave a modifying group. In some embodiments of the invention PG² isselected from t-butoxycarbonyl, allyloxycarbonyl, phthalimide, andN-benzylideneamine. In other embodiments PG² is selected fromt-butoxycarbonyl or phthalimide. In one embodiment of the invention PG²is t-butoxycarbonyl.

A “PG¹ deprotecting agent” is a reagent capable of removing the carboxyprotecting group PG¹ from the compound of Formula II during the reactingstep (b). Suitable such carboxy deprotecting agents are well-known tothe skilled person (see Greene and Wuts, supra) and may be either anacid or an alkaline solution. The concentration of the PG¹ deprotectingagent is not limited as long as it is sufficient to remove the carboxyprotecting group PG¹ and does not have an effect on the final purity oris incompatible with any container used. Preferably the PG¹ deprotectingagent is an alkaline solution. In certain embodiments the PG¹deprotecting agent is a sodium hydroxide or a potassium hydroxidesolution and in a preferred embodiment is a sodium hydroxide solution,for example of 0.5-5.0 M, preferably 0.5-2.0 M. The reacting step isenabled by closing the outlet of the SPE column so that the PG¹deprotecting agent is retained therein for a specified amount of time.The temperature and the duration of this reacting step need to besufficient to permit removal of the PG¹ carboxy deprotecting group. Incertain embodiments the reacting step is carried out at room temperatureand for a duration of between 1-5 minutes.

The step of “sending the PG¹ deprotecting agent to waste” means thatonce step (b) is complete (i.e. PG¹ is removed from the compound ofFormula II), the PG¹ deprotecting agent is allowed to pass through theSPE column and is routed out of the reaction system so that it is nolonger part of the reaction mixture. An additional benefit is that anyimpurities soluble in the deprotection solution are also routed out ofthe reaction system. The PG¹ deprotecting agent is thereforesubstantially removed from the reaction mixture for subsequent steps (d)and (e). As used herein, the term “substantially” refers to the completeor nearly complete extent or degree of an action, characteristic,property, state, structure, item, or result. For example, an object thatis substantially enclosed would mean that the object is eithercompletely enclosed or nearly completely enclosed. The exact allowabledegree of deviation from absolute completeness may in some cases dependon the specific context. However, generally speaking the nearness ofcompletion will be so as to have the same overall result as if absoluteand total completion were obtained. For example in the case of removalof the PG¹ deprotecting group, the term “substantially removed” can betaken to mean in the PG² deprotection step (e) that only sufficient PG²deprotecting agent is required to remove PG², i.e. it is not required toadd extra ion to counter the level of ion present from the PG¹deprotecting step (b).

The “elution solution” of step (d) is suitably one for which thecompound of Formula III has more affinity than it has for the solidphase. Suitably, because said compound of Formula III is relativelyhydrophilic compared with said solid phase, said elution solution is ahydrophilic solution. In some embodiments of the invention said elutionsolution is an aqueous solution and in other embodiments said elutionsolution is water.

The “PG² deprotecting agent” is a reagent capable of removing the amineprotecting group PG² from the compound of Formula III during thereacting step (e). Suitable such amine deprotecting agents arewell-known to the skilled person (see Greene and Wuts, supra) and may beeither an acid or an alkaline solution. The concentration of the PG²deprotecting agent is not limited as long as it is sufficient to removethe carboxy protecting group PG². Preferably the PG² deprotecting agentis an acid solution. A suitable acid preferably includes an acidselected from inorganic acids such as hydrochloric acid, sulfuric acidand nitric acid, and organic acids such as perfluoroalkyl carboxylicacid, e.g. trifluoroacetic acid. In certain embodiments, the PG²deprotecting agent is hydrochloric acid, and in other embodiments whenHCl is used as PG² deprotecting agent it is at a concentration of1.0-4.0M. Reacting step (e) is preferably carried out with heat to allowthe removal of PG² reaction to proceed more rapidly. The reaction timedepends on the reaction temperature or other conditions. For example,when the reacting step (e) is performed at 60° C., a sufficient reactiontime is 5 minutes.

In a preferred aspect, the [¹⁸F]-FACBC istrans-1-amino-3-[¹⁸F]-fluorocyclobutanecarboxylic acid(anti-[¹⁸F]-FACBC):

-   -   said compound of Formula II is a compound of Formula IIa:

and,

-   -   said compound of Formula III is a compound of Formula IIIa:

wherein PG¹ and PG² are as described hereinabove.

Said providing step (a) of the method of the invention may be carriedout using methods known in the art, such as for example described byMcConathy et at (2003 Appl Radiat Isotop; 58: 657-666).

Suitably, said providing step (a) comprises:

-   -   (i) reacting a precursor compound of Formula I:

-   -   -   with a suitable source of [¹⁸F]fluoride;            -   wherein:            -   LG is a leaving group;            -   PG¹ is as defined hereinabove; and,            -   PG² is as defined hereinabove;        -   to obtain a reaction mixture comprising the compound of            Formula II;

    -   (ii) applying the reaction mixture obtained in step (i) to a        solid phase so that said compound of Formula II becomes adsorbed        to said solid phase, wherein said solid phase is as defined        hereinabove.

A “precursor compound” comprises a non-radioactive derivative of aradiolabelled compound, designed so that chemical reaction with aconvenient chemical form of the detectable label occurssite-specifically; can be conducted in the minimum number of steps(ideally a single step); and without the need for significantpurification (ideally no further purification), to give the desiredradiolabelled compound. Such precursor compounds are synthetic and canconveniently be obtained in good chemical purity.

A suitable “leaving group” in the context of the present invention is achemical group that can be displaced by nucleophilic displacementreaction with fluoride ion. These are well-known in the art of syntheticchemistry. In some embodiments the leaving group of the presentinvention is a linear or branched C₁₋₁₀ haloalkyl sulfonic acidsubstituent, a linear or branched C₁₋₁₀ alkyl sulfonic acid substituent,a fluorosulfonic acid substituent, or an aromatic sulfonic acidsubstituent. In other embodiments of the invention the leaving group isselected from methanesulfonic acid, toluenesulfonic acid,nitrobenzenesulfonic acid, benzenesulfonic acid,trifluoromethanesulfonic acid, fluorosulfonic acid, andperfluoroalkylsulfonic acid. In some embodiments the leaving group iseither methanesulfonic acid, trifluoromethanesulfonic acid ortoluenesulfonic acid and in another embodiment the leaving group istrifluoromethanesulfonic acid.

In a preferred embodiment, said compound of Formula I is a compound ofFormula Ia:

-   -   and said compound of Formula II is a compound of Formula IIa:

-   -   wherein LG, PG¹ and PG² are as previously defined herein.

The “source of [¹⁸F]fluoride” suitable for use in the invention isnormally obtained as an aqueous solution from the nuclear reaction¹⁸O(p,n)¹⁸F. In order to increase the reactivity of fluoride and toreduce or minimise hydroxylated by-products resulting from the presenceof water, water is typically removed from [¹⁸F]-fluoride prior to thereaction, and fluorination reactions are carried out using anhydrousreaction solvents (Aigbirhio et at 1995 J Fluor Chem; 70: 279-87). Afurther step that is used to improve the reactivity of [¹⁸F]-fluoridefor radiofluorination reactions is to add a cationic counterion prior tothe removal of water. Suitably, the counterion should possess sufficientsolubility within the anhydrous reaction solvent to maintain thesolubility of the [¹⁸F]-fluoride. Therefore, counterions that aretypically used include large but soft metal ions such as rubidium orcaesium, potassium complexed with a cryptand such as Kryptofix™, ortetraalkylammonium salts, wherein potassium complexed with a cryptandsuch as Kryptofix™, or tetraalkylammonium salts are preferred.

In some embodiments the present invention additionally includes thefurther step (f) of purifying said reaction mixture obtained in step (e)to obtain substantially pure [¹⁸F]-FACBC.

The term “substantially” as used in “substantially pure” takes themeaning as presented above. The term “substantially pure” as used in thecontext of [¹⁸F]-FACBC encompasses completely pure [¹⁸F]-FACBC or[¹⁸F]-FACBC that is sufficiently pure to be suitable for use as a PETtracer. The term “suitable for use as a PET tracer” means that the[¹⁸F]-FACBC product is suitable for intravenous administration to amammalian subject followed by PET imaging to obtain one or moreclinically-useful images of the location and/or distribution of[¹⁸F]-FACBC.

In one embodiment, step (f) comprises:

-   -   (i) carrying out a first purification step comprising passing        said reaction mixture through a hydrophilic lipophilic balanced        (HLB) solid phase; and,    -   (ii) optionally carrying out a second purification step        comprising passing said reaction mixture through an alumina        solid phase.

In certain embodiments of the present invention said purifying step (f)can be said to consist essentially of the above-defined steps. Inparticular, the purifying step (f) as used in the present invention doesnot require that the reaction mixture is passed through an ionretardation column. This is a notable distinction over the prior artmethods where this is a required step in order to remove ions and toneutralise the reaction mixture (e.g. as described by McConathy et al,supra, and in EP-A 20172580029). As such, the method of the presentinvention is simplified over the prior art methods and as such is moresuitable for automation. In a preferred embodiment the method of theinvention is automated, and in this embodiment suitably carried out onan automated synthesis apparatus.

In another aspect of the invention is provided a system for carrying outthe method of the invention wherein said system comprises:

-   -   (a) a solid phase as defined herein for the method of the        invention;    -   (b) a source of PG¹ deprotecting agent herein for the method of        the invention;    -   (c) a source of elution solution as defined herein for the        method of the invention;    -   (d) a source of PG² deprotecting agent as defined herein for the        method of the invention;    -   (e) a reaction container; and,    -   (f) a waste means;    -   wherein said system further comprises means permitting        sequential flow from:        -   (i) (e) to (a);        -   (ii) (b) to (a);        -   (iii) (a) to (f);        -   (iv) (c) to (e) via (a); and,        -   (v) (d) to (e).

The “reaction chamber” is any vessel suitable for carrying out an ¹⁸Flabelling reaction.

The term “waste means” refers for example to a dedicated vessel intowhich is sent any components of the reaction that are no longerrequired, along with associated tubing and valves permitting thetransfer of these components away from the reaction.

In particular, the system of the invention does not comprise an ionretardation column.

In another embodiment, the system of the invention further comprises:

-   -   (g) a source of said precursor compound of Formula I as defined        herein; and,    -   (h) a source of [¹⁸F]fluoride.

In a further embodiment, the system of the present invention may alsocomprise (i) means for purifying said reaction mixture obtained in step(e) to obtain substantially pure [¹⁸F]-FACBC. Said means (i) in certainembodiments may comprise a HLB solid phase and an alumina solid phase.

The system of the invention in one embodiment consists essentially ofthe above-described features.

[¹⁸F]-radiotracers in particular are now often conveniently prepared onan automated radiosynthesis apparatus. The method of the invention maytherefore be carried out using an automated radiosynthesis apparatus. Bythe term “automated radiosynthesis apparatus” is meant an automatedmodule based on the principle of unit operations as described bySatyamurthy et at (1999 Clin Positr Imag; 2(5): 233-253). The term “unitoperations” means that complex processes are reduced to a series ofsimple operations or reactions, which can be applied to a range ofmaterials. Suitable automated synthesiser apparatus are commerciallyavailable from a range of suppliers including: GE Healthcare Ltd(Chalfont St Giles, UK); CTI Inc. (Knoxville, USA); Ion BeamApplications S.A. (Chemin du Cyclotron 3, B-1348 Louvain-La-Neuve,Belgium); Raytest (Straubenhardt, Germany) and Bioscan (Washington D.C.,USA).

Commercial automated radiosynthesis apparatus also provide suitablecontainers for the liquid radioactive waste generated as a result of theradiopharmaceutical preparation. Automated radiosynthesis apparatus arenot typically provided with radiation shielding, since they are designedto be employed in a suitably configured radioactive work cell. Theradioactive work cell provides suitable radiation shielding to protectthe operator from potential radiation dose, as well as ventilation toremove chemical and/or radioactive vapours.

Preferred automated radiosynthesis apparatus of the present inventionare those which comprise a disposable or single use cassette whichcomprises all the reagents, reaction vessels and apparatus necessary tocarry out the preparation of a given batch of radiopharmaceutical. Byuse of such cassettes the automated radiosynthesis apparatus has theflexibility to be capable of making a variety of differentradiopharmaceuticals with minimal risk of cross-contamination, by simplychanging the cassette. The cassette approach also has the advantages of:simplified set-up and hence reduced risk of operator error; improved GMP(Good Manufacturing Practice) compliance; multi-tracer capability; rapidchange between production runs; pre-run automated diagnostic checking ofthe cassette and reagents; automated barcode cross-check of chemicalreagents vs the synthesis to be carried out; reagent traceability;single-use and hence no risk of cross-contamination, tamper and abuseresistance.

In a further aspect the present invention provides a cassette forcarrying out the method of the invention on an automated synthesisapparatus wherein said cassette comprises the elements as defined forthe system of the invention.

For each aspect of the invention, features having the same name have allthe same embodiments as described in relation to other aspects of theinvention.

Brief Description of the Examples

Example 1 describes the synthesis of [¹⁸F]FACBC according to the methodof the invention.

LIST OF ABBREVIATIONS USED IN THE EXAMPLES

[¹⁸F]FACBC 1-amino-3-[¹⁸F]fluorocyclobutane-1-carboxylic acid

K222 Kryptofix 222

MeCN acetonitrileMeOH methanolQMA quaternary methyl ammoniumRCY radiochemical yieldSPE solid-phase extractionTLC thin layer chromatographyUV ultraviolet

EXAMPLES

All reagents and solvents were purchased from Merck and used withoutfurther purification. The [¹⁸F]FACBC precursor;Syn-1-(N-(tert-butoxycarbonyl)amino)-3-[[(trifluoromethyl)sulfonyl]oxy]-cyclobutane-1-carboxylicacid ethyl ester was obtained from GE Healthcare. The Oasis HLB pluscartridge and the Sep-Pak cartridges: QMA light Plus (K₂CO₃ form), tC18light, Alumina N light were purchased from Waters (Milford, Mass., USA).A Capintec NaI ion chamber was used for all radioactive measurements(model CRC15R). Radio-thin layer chromatography (radio-TLC) wasperformed on a Packard instant imager using pre-coated plates of silicagel (Merck 60F₂₅₄).

Example 1 Synthesis of [¹⁸F]FACBC

No-carrier-added [¹⁸F]fluoride was produced via the ¹⁸O(p,n)¹⁸F nuclearreaction on a GE PETtrace 6 cyclotron (Norwegian Cyclotron Centre,Oslo). Irradiations were performed using a dual-beam, 30 μA current ontwo equal Ag targets with HAVAR foils using 16.5 MeV protons. Eachtarget contained 1.6 ml of ≧96% [¹⁸O]water (Marshall Isotopes).Subsequent to irradiation and delivery to a hotcell, each target waswashed with 1.6 ml of [¹⁶O]water (Merck, water for GR analysis), givingapproximately 2-5 Gbq in 3.2 ml of [¹⁶O]water.

All radiochemistry was performed on a commercially available GE FASTlab™with single-use cassettes. Each cassette is built around aone-piece-moulded manifold with 25 three-way stopcocks, all made ofpolypropylene. Briefly, the cassette includes a 5 ml reactor (cyclicolefin copolymer), one 1 ml syringe and two 5 ml syringes, spikes forconnection with five prefilled vials, one water bag (100 ml) as well asvarious SPE cartridges and filters. Fluid paths are controlled withnitrogen purging, vacuum and the three syringes. The fully automatedsystem is designed for single-step fluorinations with cyclotron-produced[¹⁸F]fluoride. The FASTlab was programmed by the software package in astep-by-step time-dependent sequence of events such as moving thesyringes, nitrogen purging, vacuum, and temperature regulation.Synthesis of [¹⁸F]FACBC followed the three general steps: (a)[¹⁸F]fluorination, (b) hydrolysis of protection groups and (c) SPEpurification.

Vial A contained K₂₂₂ (58.8 mg, 156 μmol), K₂CO₃ (8.1 mg, 60.8 μmol) in79.5% (v/v) MeCN_((aq)) (1105 μl). Vial B contained 4M HCl (2.0 ml).Vial C contained MeCN (4.1 ml). Vial D contained the precursor (48.4 mg,123.5 μmol) in its dry form (stored at −20° C. until cassette assembly).Vial E contained 2 M NaOH (4.1 ml). The 30 ml product collection glassvial was filled with 200 mM trisodium citrate (10 ml). Aqueous[¹⁸F]fluoride (1-1.5 ml, 100-200 Mbq) was passed through the QMA andinto the ¹⁸O—H₂O recovery vial. The QMA was then flushed with MeCN andsent to waste. The trapped [¹⁸F]fluoride was eluted into the reactorusing eluent from vial A (730 μl) and then concentrated to dryness byazeotropic distillation with acetonitrile (80 μl, vial C).

Approximately 1.7 ml of MeCN was mixed with precursor in vial D fromwhich 1.0 ml of the dissolved precursor (corresponds to 28.5 mg, 72.7mmol precursor) was added to the reactor and heated for 3 min at 85° C.The reaction mixture was diluted with water and sent through the tC18cartridge. Reactor was washed with water and sent through the tC18cartridge. The labelled intermediate, fixed on the tC18 cartridge waswashed with water, and then incubated with 2M NaOH (2.0 ml) for 5 minafter which the 2M NaOH was sent to waste. The labelled intermediate(without the ester group) was then eluted off the tC18 cartridge intothe reactor using water. The BOC group was hydrolysed by adding 4M HCl(1.4 ml) and heating the reactor for 5 min at 60° C. The reactor contentwith the crude [¹⁸F]FACBC was sent through the HLB and Aluminacartridges and into the 30 ml product vial. The HLB and Aluminacartridges were washed with water (9.1 ml total) and collected in theproduct vial. Finally, 2M NaOH (0.9 ml) and water (2.1 ml) was added tothe product vial, giving a purified formulation of [¹⁸F]FACBC with atotal volume of 26 ml. Radiochemical purity was measured by radio-TLCusing a mixture of MeCN:MeOH:H₂O:CH₃COOH (20:5:5:1) as the mobile phase.The radiochemical yield (RCY) was expressed as the amount ofradioactivity in the [¹⁸F]FACBC fraction divided by the total used[¹⁸F]fluoride activity (decay corrected). Total synthesis time was 43min.

The RCY of [¹⁸F]FACBC was 62.5%±1.93 (SD), n=4.

What is claimed is: 1) A method to prepare1-amino-3-[¹⁸F]-fluorocyclobutanecarboxylic acid ([¹⁸F]-FACBC) whereinsaid method comprises: (a) providing a compound of Formula II adsorbedto a solid phase:

wherein: PG¹ is a carboxy protecting group; and, PG² is an amineprotecting group; (b) reacting said adsorbed compound of Formula II witha PG¹ deprotecting agent; (c) sending the PG¹ deprotecting agent towaste following said reacting step (b); (d) passing an elution solutionthrough said solid phase to obtain an eluted compound of Formula III:

(e) reacting said eluted compound of Formula III obtained in step (d)with a PG² deprotecting agent to obtain a reaction mixture comprising[¹⁸F]-FACBC. 2) The method as defined in claim 1 wherein said[¹⁸F]-FACBC is trans-1-amino-3-[¹⁸F]-fluorocyclobutanecarboxylic acid(anti-[¹⁸F]-FACBC):

said compound of Formula II is a compound of Formula IIa:

and, said compound of Formula III is a compound of Formula IIIa:

wherein PG¹ and PG² are as defined in claim
 1. 3) The method as definedin claim 1 wherein PG¹ is ethyl. 4) The method as defined in claim 3wherein PG² is t-butoxycarbonyl. 5) The method as defined in claim 1wherein said solid phase is a tC18 solid phase extraction (SPE) column.6) The method as defined in claim 1 wherein said PG¹ deprotecting agentis NaOH. 7) The method as defined in claim 1 wherein said PG²deprotecting agent is HCl. 8) The method as defined in claim 1—whereinsaid elution solution is water. 9) The method as defined in claim 1wherein said providing step (a) comprises: (i) reacting a precursorcompound of Formula I:

with a suitable source of [¹⁸F]fluoride; wherein: LG is a leaving group;PG¹ is as defined in claim 1; and, PG² is as defined in claim 1; toobtain a reaction mixture comprising said compound of Formula II; (ii)applying the reaction mixture obtained in step (i) to a solid phase sothat said compound of Formula II becomes adsorbed to said solid phase,wherein said solid phase is as defined in claim
 1. 10) The method asdefined in claim 9 wherein said compound of Formula I is a compound ofFormula Ia:

and said compound of Formula II is a compound of Formula IIa:

wherein LG is as defined in claim 7, PG¹ is as defined in claim 1, andPG² is as defined in claim
 1. 11) The method as defined in claim 9wherein LG is trifluoromethanesulfonic acid. 12) The method as definedin claim 1 which comprises the further step (f) of purifying saidreaction mixture obtained in step (e) to obtain substantially pure[¹⁸F]-FACBC. 13) The method as defined in claim 12 wherein step (f)comprises carrying out a first purification step comprising passing saidreaction mixture through a hydrophilic lipophilic balanced (HLB) solidphase. 14) The method as defined in claim 13 wherein step (f) furthercomprises carrying out a second purification step comprising passingsaid reaction mixture through an alumina solid phase. 15) The method asdefined in claim 1 which is automated. 16) A system for carrying out themethod as defined in claim 1 comprising: (a) a solid phase as defined inclaim 1; (b) a source of PG¹ deprotecting agent as defined in claim 1;(c) a source of elution solution as defined in claim 1; (d) a source ofPG² deprotecting agent as defined in claim 1; (e) a reaction container;and, (f) a waste means; wherein said system further comprises meanspermitting sequential flow from: (i) (e) to (a); (ii) (b) to (a); (iii)(a) to (f); (iv) (c) to (e) via (a); and, (v) (d) to (e). 17) A systemfor carrying out the method as defined in claim 9 comprising the systemas defined in claim 16 and further comprising: (g) a source of saidprecursor compound of Formula I; and, (h) a source of [¹⁸F]fluoride. 18)A system for carrying out the method as defined in claim 12 comprisingthe system as defined in claim 16 and further comprising (i) means forpurifying said reaction mixture obtained in step (e) to obtainsubstantially pure [¹⁸F]-FACBC. 19) The system as defined in claim 18wherein said means for purifying comprises a HLB solid phase andoptionally an alumina solid phase. 20) A cassette for carrying out themethod as defined in claim 1 on an automated synthesis apparatus whereinsaid cassette comprises the system as defined in claim
 16. 21) Acassette for carrying out the method as defined in claim 12 on anautomated synthesis apparatus wherein said cassette comprises the systemas defined in claim
 19. 22) The method as defined in claim 1 wherein PG²is t-butoxycarbonyl. 23) The method as defined in claim 2 wherein PG¹ isethyl. 23) The method as defined in claim 23 wherein PG² ist-butoxycarbonyl. 24) The method as defined in claim 2 wherein PG² ist-butoxycarbonyl. 25) The system as defined in claim 16 wherein saidsystem is suitable for use with an automated radiosynthesis apparatus.